The present invention relates to a fixing device used in an image forming apparatus of electrophotographic type such as a copying machine, printer or facsimile, wherein fixing is performed by heating and melting a developer image that has been transferred from a surface of an image bearing body onto a transfer sheet such as transfer paper or overhead projector transparency.
A known fixing device of this type as shown in FIG. 12 is comprised of a pair of rollers 3,4 each obtained by forming an elastic member 2 around an outer periphery of a metallic cylindrical body 1, heaters 5 for heating these rollers 3,4 and a driver 6 for rotating at least one of the rollers 3,4. The metallic cylindrical body 1 and the elastic member 2 are generally of same material for both of the pair of rollers 3,4 and they are designed so that their outer diameter and thickness of the elastic member 2 are identical. These rollers 3,4 forming a pair are abutted against each other, and a transfer sheet 8 is delivered through their contact portion (nip portion 7) wherein developer is melted at this nip portion 7 to be fixed onto the transfer sheet 8. In case materials and sizes of the pair of rollers 3,4 are identical, the nip portion 7 has a flat initial shape.
In this fixing device, the elastic member 2 of each of the rollers 3,4 is thermally deteriorated owing to heat generated by the heater 5 so that the hardness thereof is decreased. Since unevenness in components of the elastic member 2 of each of the rollers 3,4 is inevitable, the degree of decrease of hardness owing to thermal deterioration also varies between the rollers 3,4.
For instance, in case the degree of decrease of hardness owing to thermal deterioration of the elastic member 2 of the roller 3 on the upper side is larger than that of the elastic member 2 of the roller 4 on the lower side in the example of FIG. 12, the nip portion 7 forms an upwardly convex shape as indicated by the one dot chain line. On the other hand, in case the degree of decrease of hardness owing to thermal deterioration of the elastic member 2 of the roller 4 on the lower side is larger than that of the elastic member 2 of the roller 3 on the upper side in the example of FIG. 12, the nip portion 7 forms a downwardly convex shape as indicated by the two dot chain line.
In case such a deformation of the nip portion 7 owing to differences in degree of decrease of hardness owing to thermal deterioration of the elastic member 2 of the rollers 3, 4 occurs, it may cause difficult to make the transfer sheet 8 smoothly pass through the rollers 3,4 so that deficiencies in sheet delivery are generated.
Further, due to the fact that it is difficult to know variations in compositions of the elastic members 2 prior to use, it is hard to predict whether the nip portion 7 becomes upwardly convex owing to the differences in decrease of hardness owing to thermal deterioration.
On the other hand, in general, when transfer sheets are successively delivered to the fixing device, heat on the roller surface is captured by the transfer sheet. However, time lags exist between the heat capturing by the transfer sheets and heat supplying from the heaters. Thus, the temperature of the roller surface decrease at the time of starting successive delivery of the transfer sheets. When the degree of temperature decrease at the time of starting successive delivery of sheets differs between each of the pair of rollers, a difference in temperature is generated between the rollers so that this may cause deficiencies in fixing or sheet delivery.